Apparatus for monitoring film thickness by reflecting a light beam from the film surface

ABSTRACT

This disclosure is directed to a nondestructive test system which makes use of a reflected light beam to determine the thickness of a film deposited onto a substrate within a vacuum system. The thickness of the film is monitored during deposition without any deleterious effects on the film deposition. The system may likewise be used for relative position of one part with respect to another part.

United States Patent Patten 1 Feb. 29, 1972 [54] APPARATUS FORMONITORING FILM THICKNESS BY REFLECTING A LIGHT BEAM FROM THE FILMSURFACE [72] lnventor: Raymond A. Patten, 8612 Madison Place,

Oxon Hill, Md. 20022 [22] Filed: Sept. 25, 1970 [21] Appl. No.: 75,520

[52] US. Cl ..356/1, 250/219 Tl-l, 356/156 [Sl] Int. Cl ..G0lb 11/06,GOlb ll/l4 [58] Field ofSearch ..356/1, 156; 250/2l9 TH [56] ReferencesCited UNITED STATES PATENTS 3,016,464 1/1962 Bailey....,...

3,017,512 l/l962 Wolbert ..250/2l9 TH UX Primary Examiner-Ronald L.Wibert Assistant Examiner-Orville B. Chew, Il

Attorney-R. S. Sciascia, Arthur L. Branning, .l. G. Murray and M. L.Crane [57] ABSTRACT This disclosure is directed to a nondestructive testsystem which makes use of a reflected light beam to determine thethickness of a film deposited onto a substrate within a vacuum system.The thickness of the film is monitored during deposition without anydeleterious effects on the film deposition. The system may likewise beused for relative position of one part with respect to another part.

10 Claims, 4 Drawing Figures PATENIEumzs m2 3, 645,623

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{6 30 33 28 Q DETECTOR DETECTOR l l 34' 35 as CHOPPER LOCK IN MULTIPLIERAMPLIFIER RECORDER 3 INVENTOR.

RAYMOND A. PATTE/V APPARATUS FOR MONITORING FILM THICKNESS BY REFLECTINGA LIGHT BEAM FROM THE FILM SURFACE STATEMENT OF GOVERNMENT INTEREST Theinvention described herein may be manufactured and used by or for theGovernment of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION This system is directed to measurement offilm thickness deposited onto a substrate and more particularly to anondestructive system which measures the film thickness within a vacuumsystem while the film is being deposited.

Various methods have been used in the past to measure film thickness.Some of these measure the film within the vacuum system duringdeposition, others measure thickness after removal from the vacuumsystem. X-ray methods require the use of specialized equipment and atrained operator. Such a system depends on an accurate knowledge of theattenuation coefficient and generally are not usable as a continuousmonitor of film thickness while it is growing.

Quartz crystal oscillators have been used. These systems continuouslymeasure the film thickness; however, the film measured is the filmdeposited onto a portion of the quartz crystal and not the filmdeposited onto the substrate of interest which may be different.

Other systems such as interferometric methods, optical densitytechniques, and ellipsometry methods, which is an optical system, areaccurate, nondestruct test systems; however, each have their drawbacksand limitations.

SUMMARY OF THE INVENTION The system of this invention provides anondestructive means of measuring the physical thickness of a film ofmaterials deposited within a vacuum system. The system uses two distinctlight beams which are formed by a grid and chopper and imaged ontoseparate halves of the substrate and reflected therefrom in parallelbeams at an angle of 85.5. The light reflected from the substrate isdirected through a fixed grating by a suitable optical imaging systemand directed onto a light detector. The light of each beam reflected bythe substrate subtracted such that any difference will be proportionalto the thickness of the film. Therefore, one-half of the substrate ismasked such that the film is deposited onto only one-half of thesubstrate. As the film builds up, the light reflected by the side uponwhich the film is deposited will be moved laterally, Lateral movement ofthe light causes a displacement in the reflected light beam. Therefore,a portion of the displaced reflected light will be blocked by the fixedgrating. Blocking of a portion of the light will produce a difference inthe intensity of the two beams emerging from the grating and beingdetected. By subtracting the light intensity of one beam from the other,a resultant signal proportional to the film thickness is generated.Means is also provided to compensate for variations in source intensityand surface reflectivity.

STATEMENT OF THE OBJECTS It is therefore an object of the invention toprovide a system for measuring film thickness of a material deposited ona sub strate within a vacuum system without the requirement ofspecialized equipment.

Another object is to be able to measure film thickness regardless of itselectrical or optical properties.

Yet another object is to measure film thickness while compensating forvariations in source intensity and surface reflectivity.

I Still another object is to provide a system which senses the actualthickness of the film deposited onto a substrate.

While still another object is to provide a system which is simple inoperation and may be operated by unskilled as well as skilled personnel.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a diagrammatic view of thesystem.

FIG. 2 illustrates a grid through which separate beams of light arepermitted to emerge to form two separate beams of light.

FIG. 3 illustrates a block diagram of an electrical circuit used in thedetector system, and

FIG. 4 illustrates a tuned fork used as a light chopper.

DESCRIPTION OF THE SYSTEM Now referring to the drawing, there is shownby illustration a diagrammatic view illustrating the relative parts ofthe system for determining thickness of a film deposited onto a portionof a substrate. The substrate 10 is coated in any well known evacuatedsystem such as a bell jar 11 shown in dotted line. Included within thebell jar is a mask 12 suitably posi tioned to prevent deposition of afilm onto one-half of the substrate. Therefore, a film of a material isdeposited onto only one-half 13 of the substrate with the other half 14free from any coating. The halves are shown separated by a dotted line.

The system for measuring the thickness is shown entirely on the outsideof the evacuated system, therefore, the vacuum system is not affected bythe film-measuring system. The system includes a DC power source 15 thatoperates a light source 16 such as a 2-watt tungsten filament to providea light beam which is directed through a condenser 17 formed of two 39mm. diameter 63 mm. focal length achromats 18 and 19. The converginglight emerging from the lens 19 forms an image of the filament of thelight source at a mirror 21 which is located at the focal point of alens 22 and so positioned to direct the light toward lens 22 and sopositioned to direct the light toward lens 22 and the substrate 10. Thelight from the condenser 17 is directed toward a mask 23, shown in FIG.2, which has two openings therein that divides the light beam into twoseparate beams. The mask includes the space grids 24 and 25 throughwhich the light passes. The mask and grids and optical system arelocated such that converging light passing therethrough forms a reducedimage at the midplane of the substrate. A light chopper 26 in the formof a tuning fork, tuned at Hz., illustrated in FIG. 4, is positionedopposite the mask 23 and operative to alternately block one light beamand then the other beam by use of an attached shield 30. The light beamwhich is not blocked is reflected by mirror 21 through lens 22 onto thesubstrate. The light passing through grid 24 is optically directed ontoand reflected by substrate half 13 while the light that passes throughgrid 25 is optically directed onto and reflected by substrate half 14.The light is directed from mirror 21 onto lens 22. Lens 22 is an 173.5,35 mm. focal length Cooke triplett from which the light emerges in aparallel beam. The light is directed onto the substrate such thatwithout a film deposited thereon, the light will be reflected by thesubstrate at an angle of 85.5. The light reflected by the substrate isincident on a lens 26 which is identical with lens 22, from which thelight emerges in a convergent beam. The light from lens 26 is directedonto partially reflecting mirror 27 which permits a small portion of thelight to pass therethrough. The light passing through mirror 27 isincident on a photodetector 28 such as silicon photodetectors. The lightreflected by mirror 27 is directed through a fixed adjustable grid 31onto a condenser 32 which is the same as the condenser 17. The lightpassing through the condenser 32 is incident on a photodetector 33 whichconverts the light into an electrical signal. The grids 24, 25, and 31are 50 lines/inch Ronchi rulings.

Prior to deposition of a film, the system may be calibrated by operationof the detector system. Without any film deposited onto the substratethe output signal should be zero, since the intensity of the two beamsshould be the same and there is no difference which is required toobtain an output signal. In like manner, additional films could beapplied to a surface which already has a film deposited thereon byadjusting the system at the beginning such that the output is zero.Subsequent thereto, deposition of a film onto the substrate will bringabout a difference the same as if there had been no film deposited ontothe substrate at the start. Since the system measures the light thatpasses through grid 31, the calibration may be carried out by adjustingthe grid 31 horizontally until the intensity of the two light beamspassing through the grid are equal as indicated by the output signal.Subsequently, any additional film deposited onto one-half of thesubstrate will displace one of the beams and the system will indicatethe difference in the light intensity of the two beams passing throughthe grid 31. The system may also be calibrated electrically by adjustingthe phase of the lock-in amplifier.

The light that passes through grid 31 is incident on a photodetector 33which produces an electrical output signal corresponding to theintensity of the incident light. The electrical signal is directed intoan electrical multiplier circuit 34 from which the output signal isdirected into a lock-in amplifier 35. The signal from the lock-inamplifier is directed into a recorder or indicator which records orindicates the value of the output signal from the lock-in amplifier. Thelock-in amplifier derives a signal from the chopper such that thelock-in amplifier is controlled by the frequency of the chopper. Assuch, a signal resulting from one light beam due to operation of thechopper will correspond with one phase of operation of the lock-inamplifier and the signal resulting from the other light beam due tooperation of the chopper will correspond with the other phase ofoperation of the lock-in amplifier. The signal output produced by thelock-in amplifier is equal to the difference in the signals receivedduring each half cycle of operation of phase change.

ln some instances, variations may occur in source intensity and/orsurface reflectivity, therefore, it may be necessary to provide meansfor compensating for such changes. The light detector 28 has beenprovided to detect the light passed by partially reflective mirror 27.The signal produced by detector 28 is suitably amplified and directedinto the multiplier 34 where the signal from detector 28 is divided intothe signal from detector 33 directed into the multiplier. The resultantsignal from the multiplier is directed into the lock-in amplifier.Adjustment of the phase of the lock-in amplifier allows the signal ofone light beam to be subtracted from a signal due to the other lightbeam whereby the resultant signal is directly proportional to the filmthickness. The resultant signal of the lock-in amplifier provides adetermination of film thickness whether or not the light detector 28 isprovided.

Once the system is set up and ready for operation, detection of filmthickness may be carried out as follows The film deposition means may beoperated to start depositing a film onto the substrate. The lightsource, the chopper and associated electronic equipment are madeoperative to start detection of the film. The chopper vibrates toalternately block one light beam and then the next light beam thatpasses through grids 24 and 25. The light that passes through grid 24 isreflected by substrate half 13 (the half that is being coated) and thelight passing through grid 25 is incident on substrate half 14, (thehalf not being coated). Each light beam is reflected by the respectivesubstrate halves alternately onto lens 26 and onto partially reflectivemirror 27. Part of the light pmes through mirror 27 and is detected bylight detector 28, the remainder of the light is reflected by mirror 27through grid 31 and light condenser 32 onto light detector 33.

The light reflected by substrate half 13 upon which the film is beingdeposited will be displaced laterally an amount equal to the thicknessof the film, therefore, the light beam will be displaced relative to thegrid 31. Since the one light beam is displaced with respect to grid 31,less light in that beam will pass through the grid, consequently thelight for that beam incident on the detector will be less than thatprior to deposition of the film. The light beam incident on substratehalf 14 will not be affected, therefore, the light beam that passesthrough grid 25 and grid 31 will not be affected and the intensity willbe unchanged. The outputs from the photodetector 33 are directed intothe lock-in amplifier where the signal from one beam is subtracted fromthe other. The resultant output signal from the lock-in amplifier isdirectly proportional to the film thickness.

It has been noted above that some of the light passes through partiallyreflective mirror 27 onto a photodetector 28. The signal output fromphotodetector 28 may be divided into the signals from detector 33 tonormalize any variations in source intensity and surface reflectivity.The resultant signals are fed into the lock-in amplifier as set forthabove for the signals from detector 33.

The film thickness measuring system has been shown on the outside of thevacuum deposition chamber, therefore, the walls of the chamber orwindow" in the area through which the light passes should be such thatthe wall does not interfere optically with the light that passestherethrough. To avoid any optical interference with the light due topassing through the windows in the vacuum chamber, the lenses 22 and 26may be formed in the chamber wall and properly aligned to direct lightonto the substrate and receive the light reflected from the substrate inthe manner described above.

The system has been shown with mirrors 21 and 27 which fold the light inan optical path. Such a system could be placed within the vacuum chamberso that the light need not pass through the walls of the vacuum chamber.The system could be built without the mirrors 21 and 27 positioned asthey are. The requirement is that the parallel rays of light be incidenton the two sections of the substrate and reflected into an opticalsystem that detects the light.

The system has been described using a single substrate upon which a filmis to be deposited. The system could be used with two separate surfacespositioned side by side with their surfaces in parallelism where onelight beam is directed onto one surface and the other light beam isdirected onto the other surface. Such an arrangement may be used notonly to measure deposition of a film onto one surface but may be used tomeasure relative movement of one surface with respect to the othersurface.

In addition to the above, the system may be used to measure pressure.Pressure measurement may be made by having a fixed surface and a movablesurface which moves laterally with respect to the fixed surface due topressure on the one surface.

in any of the above-described systems, the surfaces upon which the twoseparate light beams are incident need not be exactly on the same level.Differences of level may be taken into account optically by adjustmentof the grid 31 which permits the light to pass therethrough to thedetector 33 or electronically by changing the phase of the lock-inamplifier.

Thus, it is seen that the above-described system may be used formonitoring film thickness, single or multiple, for determining relativemovement of two surfaces or for determining pressure in a structurewhich has a moveable head.

Obviously many modifications and variations of the present invention arepossible on the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed and desired to be secured by Letters Patent of theUnited States is:

1. A monitor for determining the position of a first surface relative toa second surface, which comprises:

first and second optical lenses positioned in optical alignment witheach of said surfaces,

said first lens so positioned and operative to direct parallel rays oflight onto each of said surfaces, said second lens so positioned andoperative to receive parallel rays of light reflected by each of saidsurfaces,

means in optical alignment with said first lens operative to alternatelydirect first and second separate light beams through said first lenswith said first beam directed onto said first surface and said secondbeam directed onto said second surface,

a first light detector means,

grid means in optical alignment with said second lens,

said grid means adjustable relative to said light beams to restrictpassage of said beams through said grid to said first detector means,

a lock-in amplifier means,

said light detector alternately directing an output signal into saidlock-in amplifier in accordance with the intensity of said first andsecond light beams received thereby,

said lock-in amplifier producing an output signal representing anydifference in the light intensity of said first and second light beams,and

means for receiving said output signal from said lock-in amplifier andindicating a measure of said output signal which is proportional to therelative positions of said two surfaces.

2. A monitor as claimed in claim 1 which comprises:

a light chopper means for alternately blocking said first and secondlight beams.

3. A monitor as claimed in claim 2 in which:

said light chopper is synchronized with said lock-in amplifi- 4. Amonitor as claimed in claim 3 which includes:

a reflective mirror to reflect said first and second light beams ontosaid first lens and a second light detector,

a partially reflective mirror for transmitting a portion of saidreflected light beams onto said second light detector and reflecting theremaining light through said grid means in optical alignment with saidsecond lens.

5. A monitor as claimed in claim 4 which includes:

a light mask,

said light mask including a pair of grids therein through which lightpasses,

said mask operative to form said first and second beams of light.

6. A monitor as claimed in claim 5 in which:

said first light beam is directed onto and reflected by said firstsurface and said second light beam is directed onto and reflected bysaid second surface.

7. A monitor as claimed in claim 6 which includes:

a multiplier circuit,

said first and said second light detectors directing their outputsignals into said multiplier circuit,

said multiplier circuit dividing said signal from said second detectorinto the signal from said first detector, and

said multiplier circuit directing its output signal into said lock-inamplifier.

8. A monitor as claimed in claim 1 wherein:

said first and second surfaces are formed by a single substrate, wherebysaid monitor determines the thickness of a film deposited onto saidfirst surface portion of said substrate during deposition of a filmthereon.

9. A monitor as claimed in claim 1 wherein:

said first surface is a movable head of a pressure device and saidsecond surface is fixed relative to said movable head with theirsurfaces in parallelism whereby said monitor determines pressure bymeasuring any movement of said movable head.

10. A monitor as claimed in claim 1 wherein:

said first and second surfaces are separate side-by-side surfaces withtheir surfaces in parallelism, whereby said monitor determines therelative movement of said first surface relative to said second surface.

1. A monitor for determining the position of a first surface relative toa second surface, which comprises: first and second optical lensespositioned in optical alignment with each of said surfaces, said firstlens so positioned and operative to direct parallel rays of light ontoeach of said surfaces, said second lens so positioned and operative toreceive parallel rays of light reflected by each of said surfaces, meansin optical alignment with said first lens operative to alternatelydirect first and second separate light beams through said first lenswith said first beam directed onto said first surface and said secondbeam directed onto said second surface, a first light detector means,grid means in optical alignment with said second lens, said grid meansadjustable relative to said light beams to restrict passage of saidbeams through said grid to said first detector means, a lock-inamplifier means, said light detector alternately directing an outputsignal into said lock-in amplifier in accordance with the intensity ofsaid first and second light beams received thereby, said lock-inamplifier producing an output signal representing any difference in thelight intensity of said first and second light beams, and means forreceiving said output signal from said lock-in amplifier and indicatinga measure of said output signal which is proportional to the relativepositions of said two surfaces.
 2. A monitor as claimed in claim 1 whichcomprises: a light chopper means for alternately blocking said first andsecond light beams.
 3. A monitor as claimed in claim 2 in which: saidlight chopper is synchronized with said lock-in amplifier.
 4. A monitoras claimed in claim 3 which includes: a reflective mirror to reflectsaid first and second light beams onto said first lens and a secondlight detector, a partially reflective mirror for transmitting a portionof said reflected light beams onto said second light detector andreflecting the remaining light through said grid means in opticalalignment with said second lens.
 5. A monitor as claimed in claim 4which includes: a light mask, said light mask including a pair of gridstherein through which light passes, said mask operative to form saidfirst and second beams of light.
 6. A monitor as claimed in claim 5 inwhich: said first light beam is directed onto and reflected by saidfirst surface and said second light beam is directed onto and reflectedby said second surface.
 7. A monitor as claimed in claim 6 whichincludes: a multiplier circuit, said first and said second lightdetectors directing their output signals into said multiplier circuit,said multiplier circuit dividing said signal from said second detectorinto the signal from said first detector, and said multiplier circuitdirecting its output signal into said lock-in amplifier.
 8. A monitor asclaimed in claim 1 wherein: said first and second surfaces are formed bya single substrate, whereby said monitor determines the thickness of afilm deposited onto said first surface portion of said substrate duringdeposition of a film thereon.
 9. A monitor as claimed in claim 1wherein: said first surface is a movable head of a pressure device andsaid second surface is fixed relative to said movable head with theirsurfaces in parallelism whereby said monitor determines pressure bymeasuring any movement of said movable head.
 10. A monitor as claimed inclaim 1 wherein: said first and second surfaces are separateside-by-side surfaces with their surfaces in parallelism, whereby saidmonitor determines the relative movement of said first surface relativeto said second surface.